Our aim is to study the organization of neurotransmitter receptors on nerve and muscle cells in relationship to the development and function of synapses. Our recent work has focused upon the factors, extinsic and intrinsic to the developing skeletal muscle fiber, which regulate the distribution of nicotinic acetylcholine receptors. Acetylcholine receptor aggregation in induced on cultured myotubes by neuronal factors, and this system is used to study the mechanisms of receptor aggregation, as well as the stabilization or elimination of aggregates which occur in developing neuromuscular junctions. In experiments using image intensification to directly observe changes in receptor distribution, and electron microscopy to study changes in the subsurface cytoskeleton and extracellular matrix, we have demonstrated discrete steps in the assembly of receptor aggregates from diffuse receptors. The transition from the clouds of microaggregates, which first appear, to large, dense aggregates is temperature dependent and involves an increase in stability, as seen when the aggregation factor is removed or sodium azide is added to reduce ATP formation. This transition is correlated with the appearance of specific surface structures. In microaggregates, there are characteristic mounds in the cell surface, subtended by loosely organized cytoplasmic filaments. The aggregates have, in addition, an increased association with basal lamins, and a characteristic dense filamentous structure below the cell membrane. Factor-induced receptor aggregates develop optimally at 36C, are rapidly but reversibly destabilized at 38C, and are more stable at 24C.